To Set, check or remove Read receipts on Mac Mail:-
Open terminal :-
Adjust this command to suit your requirements copy and paste into terminal then press enter :-
defaults write com.apple.mail UserHeaders '{"Return-Receipt-To" = "your_email@somewhere.com"; "Disposition-Notification-To" = "your_email@somewhere.com"; }’
Copy the above command and paste it into terminal and click enter.
To check read receipt status, copy and paste this command in terminal , press enter:-
defaults read com.apple.mail UserHeaders
To remove read receipt from Mac Mail, copy and paste this command in terminal and press enter.:-
defaults delete com.apple.mail UserHeaders
Regards.